Previous activated sludge methodology has used various continuous flow configurations comprising a primary solids settlement tank, an aeration tank and a secondary solids settlement tank. Solids are removed from the secondary solids settlement tank and returned to the aeration tank in order to maintain a stable solids concentration therein. Treatment of wastewater has also been achieved in intermittently aerated activated sludge systems both with and without primary solids settling. With all activated sludge configurations, their process efficiency is drastically affected by sludge bulking conditions, a situation where activitated sludge solids exhibit a low zone settling velocity, sufficient to cause these solids to be lost from the treatment system. This loss of solids and associated treatment process upsets take considerable time to rectify.
For a biological treatment process in general and particularly the activated sludge process to be effective and economically viable, it is essential that the biomass exhibit good settling properties in order that efficient gravitational solids-liquid separation can be practiced. A number of circumstances can arise which have been reported to contribute to the generation of a non-flocculating biomass (bulking sludge) which has poor settling properties and thus impose an impediment to the cost effectiveness of the activated sludge process as a method of biological wastewater treatment. The number and type of microorganisms which can contribute to or cause a non-flocculating biomass are numerous and quite prolific. The growth of such organisms is known to be associated with the treatment of domestic wastes, readily degradable high strength predominately carbohydrate type of wastes such as those generated in the food, potato, milk, brewing and similar processing industries, or an admixture of such wastes in any proportion.
A number of approaches have been suggested since the activated sludge process was initially developed, in order to alleviate sludge bulking conditions, the majority having been modifications to the conventional continuous feed continuously aerated configuration with separate clarifier and continuous return activated sludge. Some of these include tapered feed and aeration combinations, the use of aerobic and anoxic subreactors or zones, initial small volume anoxic, aerobic or anaerobic reactors for contacting the return activated sludge with the influent waste flow and various chemicals to selectively limit or kill the growth of non-flocculating portion of the biomass. The success of these modifications and variations has been varied and often has produced inconsistent results.
In order to achieve best possible solids-liquid separation it is necessary that the biomass contain a relatively small portion of the non-flocculating type of organisms. This enhances the ability of the biomass to entrap coarse, fine and colloidal particulate matter which, if not removed in the solids-liquid separation unit operation, requires a separate costly filtration or other type of process unit operation for its removal. The nature of the non-flocculating biomass enables strong solid bridging mechanisms, with high intra solid attractive forces assisted by micro particulate biocoagulation involving extra cellular polymer compounds, to take place within the biomass. The absence of some non-flocculating microorganisms in the biomass leads to pin-point type of sludge or to a type that results in a turbid liquid layer during and following the settling operation. This also means that the efficiency of the process is reduced necessitating the addition of other processes or unit operations to remedy the deleterious situation. One solution to poor solids-liquid separation has been to increase the area and liquid depth of the solids-liquid separation unit and thus the hydraulic retention time of the unit. There is a limit to the hydraulic retention time that can be used in practice due to anaerobic and/or anoxic biological transformations which can take place within the biomass. Too low a solids flux combined with too long a period whereby the biomass is in a non-aerobic condition only leads to a further loss in process efficiency and cost effectiveness due to the need to use additional processes or unit operations.
In conventional activated sludge wastewater treatment methodology two flow configurations can be described, complete mix or plug-flow. Tracer studies to determine hydraulic residence time distributions and dispersion number characteristics, a dimensionless number describing diffusive mixing and transport, essentially describe the flow predominance of the configuration. A dispersion number of or near to 0 essentially describes a plug-flow configuration while a large value of the dispersion number, approaching infinity, describes an essentially completely mixed configuration. Activated sludge systems operating, or predominantly operating, in the complete mix configuration are very prone to generation of sludges which bulk and which are identified as having a low zone settling velocity i.e. poor solids-liquid separation. Such configuration is specially unsuited to the treatment of readily degradable food processing types of wastes or to domestic wastewaters where a high level of ammonia removal is required. In such cases bulking sludge or biomass exhibiting poor solids-liquid separation severely limits the efficacy of the process.
The hydraulic residence time distribution can also be used to fit various hydraulic models, which also describe the degree of plug-flow, in the form of a certain number of smaller completely mixed reactors connected in series which in total exhibit the plug-flow behavior. An equivalent four reactors in series is known to approximate a plug-flow hydraulic configuration. Added to this type of model, is the ability to be able to describe, bypass, backmix, bypass and dead volume fractions in the flow configuration.
Wastewaters are characteristically described by parameters which quantify their oxygen consuming potential, solids content and the availability of other essential nutrients necessary for a healthy and efficiently operating biological treatment process. The concentration terms in domestic wastewaters are a function of the volume ratio of water that is used to transport the wastes to the treatment facility, the diet of the population contributing to the system and the residence time of the combined water and wastes in the sewerage system. The principal treatment parameters are hence the carbonaceous oxygen consuming fraction (or organics) variously determined as BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), PV (Permanganate Value), TOC (Total Organic Carbon) and the nitrogenous oxygen consuming fraction described as TKN (total Kjeldahl nitrogen), organic nitrogen, ammonia nitrogen or free and saline ammonia, and nitrite nitrogen. These parameters can be used as a measure of the soluble, colloidal and particulate forms and their various fractions. For example, domestic wastewaters may have an associated BOD and a suspended solids content ranging from some 350 mg/L to a low value of about 90 mg/L for each parameter depending on the amount of water in the carrier system. TKN is similarly variable from about 80 mg/L to lower concentrations. On a practical basis particulate matter contributes to about 50 percent of the BOD (or other parameters). The soluble and colloidal matter makes up the other 50 percent with the colloidal fraction contributing about 15 percent. Domestic sewage may contain less then about 60 mg/L of soluble BOD (or other parameter).
The characteristics of industrial wastewaters are a function of the process they are derived from. Industrial wastewaters from the food industries, such as potato, milk, vegetable, brewery industries have a high BOD (or other parameter) in which the soluble fraction also leads to a high BOD (or other parameter) in the range of 200 to 2000 mg/L. Wastewaters having a total BOD (or other equivalent parameter) in excess of about 3000 mg/L are generally not economically amenable to biodegradation by an aerobic process alone such as the activated sludge process. For the purpose of demonstrating this invention a typical domestic wastewater having a total BOD of about 300 mg/L will be considered; however, the invention and its application is not limited to this strength or type of waste.
It is generally observed that BOD removal by the activated sludge and other biological processes takes place by storage, synthesis and oxidation mechanisms in which all three processes occur simultaneously. These assumed basic mechanisms can be enhanced, the extent of which is determined by the net hydraulic flow configuration. Absorption of soluble substances (organics) by microorganisms is thought to take place by enzyme transport together with diffusive mechanisms. It is a rapid interaction the amount of which is determined by the ability of the active organisms of the biomass to absorb; the latter is functional on the population of intra and extra cellular enzymes that are available, the fraction of receptive transport sites or centers associated with the microorganisms, the fraction of previously absorbed material that remains in an unmetabolized state, i.e. the concentration driving forces, all of which are related to the active mass fraction of the biomass. The active mass fraction has sometimes been described as the viable fraction or the degradable fraction. This invention provides a process whereby the degradable fraction property of biomass and its ability to absorb soluble substrate is maximized.
The transport of soluble substrate (organics) by enzymatic mechanisms, or absorption, is an energy intensive reaction, the magnitude of which can be shown by the specific oxygen uptake rate (SOUR) of the biomass before and after substrate contact. Synthesis reactions and cellular growth do not necessarily occur on the onset of absorption. Cellular growth mechanism do not begin to function for some considerable time lag after absorptive transport saturation as can be evidenced by cellular Adenosine Tri-Phosphate (ATP) concentrations.
A typical bioresponse profile during and following absorption is described in FIG. 1. Two situations are shown in FIG. 1, one where aeration of the biomass is in the presence of residual unabsorbed substrate (curve I) and one where aeration of the biomass takes place in the absence of residual unabsorbed susbstrate (curve II). The second bioresponse was obtained by centrifugal separation of biomass and substrate whereby the volume of removed substrate was replaced with distilled water containing a supply of necessary nutrients. Bioprofile I therefore describes the oxygen mass required for substrate absorbed together with biodegradation of residual substrate not initially absorbed. Bioprofile II essentially describes the oxygen mass required to metabolize initially absorbed substrate. The mechanism of absorption is quite different from that of adsorption. Adsorption is a surface attraction phenomena which only accounts for very minimal substrate removal on contact with biomass. A maximum of about 3 to 5 percent under optimum circumstances can be realized. On the other hand, absorption of soluble substrate of up to 90 percent at practical floc-loading can be achieved.
It has been mentioned that the absorption potential or efficiency of a biomass is measurable by the initial magnitude of and the resulting elevation of the specific oxygen utilization rate (SOUR) of that biomass. The absorption potential of a biomass is also functional on the fraction of that biomass that is active and degradable, that latter being determined by the organic loading or mean cell residence time of the biomass. This is graphically depicted in FIG. 2.
The magnitude of the elevation in the SOUR bioprofile is also dependent on the initial substrate to biomass ratio termed the floc-load (F). This is shown schematically in FIG. 3. Units of floc-load are described as mg BOC, COD, TOC, PV per g biomass. Biomass may be described as mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids, (MLVSS). The degradable mass of either MLSS or MLVSS on the surface or near surface layers of biomass in an attached growth system expressed in the same terms of oxygen utilization provides a measure of the substrate saturation capacity of that particular biomass. The initial absorption mechanism is quick, the major fraction of substrate transport taking place in only about 10 to 20 minutes. Little absorption takes place after about 45 minutes. Therefore the absorption mechanism results in a reduction in substrate concentration as shown generally in FIG. 4.
The percentage removal and floc-load relationship is waste specific and also functional on the active or degradable fraction of the biomass and the relative magnitude of SOUR existing on initiation of absorption mechanisms with respect to the magnitude associated with viable biomass in a nonabsorption state viz initial value of SOUR. The absorption potential of a biomass is limited in the first instance, as shown in FIG. 5, by the active or degradable fraction of the biomass, by the floc-load and by the availability of receptor transfer sites or storage capacity for that degradable fraction of biomass.
Extended periods of aeration of the contacting biomass or contacting of biomass which has experienced prolonged periods without the presence of oxygen reduces the absorption potential of that biomass, as depticted in FIG. 6.
The magnitude of percent of substrate absorbed, relative to the criteria described above, at a specific floc-load is instrumental in determining the sludge settling characteristics of the biomass following conventional aeration periods.
For some time now it has been recognized that there is a need to improve activated sludge wastewater treatment methodology, for both domestic and high soluble BOD industrial wastewaters, in order to be able to economize on both capitol equipment and operating costs. This may be brought about by manipulating and optimizing the biological processes in order to provide consistent degradation together with consistent and good sludge settlement behavior.
The following patents and publications provide various background information and details of certain apparaus, systems and processes which may be used in connection with the present invention.
U.S. Pat. Nos. 2,852,140, MacLaren, 3,053,390, Wood, 3,202,285, Williams, 3,415,378, Fukuda, and 3,433,359, Lundin et al. Tank structure and aeration apparatus are disclosed.
U.S. Pat. No. 3,264,213, Pav. Describes an activated sludge process.
U.S. Pat. No. 3,524,547, Nicol. A sewage treatment plant utilizing aeration is described, which includes an inlet compartment and two treatment compartments. Sewage from the inlet compartment is transferred substantially contemporaneously from the inlet compartment into the first of the treatment compartments and from first treatment compartment to the second treatment compartment and treated sewage is removed from the second treatment compartment. The flow is then reversed, flow being from the inlet compartment to the second treatment compartment to the first compartment and then from the system. The two treatment compartments thus serve, alternately, as the first and second treatment stages.
U.S. Pat. No. 3,530,990, Grimshaw. An aeration system and apparatus is described which, if desired, could be adapted to provide for aeration in the present invention. Aeration apparatus per se is not part of the present invention, however, and any aeration apparatus may be adapted for use in the present invention.
U.S. Pat. No. 3,732,160, Klock. A fixed film digestion material, referred to as a "filter", which presents absorption media to the sewage is described. The substrate for the film may be plates, spheres, etc.
U.S. Pat. No. 3,805,957, Oldham. Tank structure, aeration and circulation are described.
U.S. Pat. No. 4,069,147, Abrams, et al. Multiple tank system with aeration is described.
U.S. Pat. No. 4,081,368, Block et al. A multiple compartment activated sludge treatment system with recycle of activated sludge is described.
U.S. Pat. No. 4,206,047, Mandt. A multi-stage waste water treatment system involving aeration and mixing, utilizing pure oxygen is described.
U.S. Pat. No. 4,152,259, Molvar. Aeration apparatus.
U.S. Pat. No. 4,290,887, Brown, et al. A weir arrangement for decanting treated wastewater may optionally be used with the present invention; however, any decanting apparatus may be used.
U.S. Pat. No. 4,468,327, Brown, et al. A single vessel treatment process is described. Aeration, decanting and various other apparatus may be used with the present invention. Reference is made to the subject patent for a discussion of various aspects of the activated sludge digestion process which, in a very broad sense, relates to the present invention. U.S. Pat No. 4,468,327 utilizes a single reaction vessel and the process is carried out with continuous inflow and intermittent aeration and decanting. In contrast, the present invention is a multiple reaction vessel process in which the process variables differ substantially from the process of the subject patent.
OXIDATION DITCHES IN WASTEWATER TREATMENT, D. Barnes, C. F. Forster, D. W. M. Johnstone, Pitman Press, Bath, Avon, U.K.
"Development of the Passveer Extended Aeration System", Batty, J. A., Goronszy, M. C., Clarke, R., The Shire and Municipal Record (Australia), November, 1974.
"Continuous Intermittent Wastewater Systems for Municipal and Industrial Effluents", Barnes, D. and Goronszy, M. C., Oxford District Centre, Oct. 4, 1979, Institute of Public Health Engineers, London.
"Control of Activated Sludge Filamentous Bulking", Chudorba, J., Grau, P., Ottova, Blaha, J., Madera, V., Water Research; Part I, Vol. 7, pp 1163-1182; Part II, Vol. 7, pp 1389-1406; Part II, Vol. 8, pp 231-237 (1973).
"Single Vessel Activated Sludge Treatment for Small Systems", Goronszy, M. C., Journal WPCF, Vol. 51, pp 274-287; presented Oct. 6, 1977, 50th Annual Conference of the Water Pollution Control Federation, Philadelphia, Pa.
"The Activated Sludge Process: State of the Art", W. Wesley Eckenfelder, Jr., et al., CRC Critical Reviews in Environmental Control, Volume 15, Issue 2, 1985.
"PRINCIPLES OF WATER QUALITY MANAGEMENT", W. Wesley Eckenfelder, Jr., CBI Publishing Company, Inc., Boston (1980).
The foregoing publications describe the basic theory and operation of the activated sludge process and the extended aeration process for treatment of sewage.